[18BF] Can only run train to London if running to own token

lib/engine/game/g_18_bf/game.rb

@@ -146,6 +146,19 @@ def revenue_for(route, stops)
             end
           super + bonuses
         end
+
+        def check_other(route)
+          check_london(route)
+        end
+
+        def check_london(route)
+          # Can only run to London if running to your own token.
+          london_stops = route.visited_stops & @london_cities
+          return if london_stops.all? { |city| city.tokened_by?(current_entity) }
+
+          raise GameError, 'Route may not include London unless running to a ' \
+                           "#{current_entity.id} token."
+        end
       end
     end
   end

lib/engine/game/g_18_bf/map.rb

@@ -279,6 +279,9 @@ def setup_london_hexes
           london = @hexes.find { |hex| hex.coordinates == LONDON_HEX_CENTRE }
           london.neighbors.clear
           @london_zoomed << london
+
+          # Keep references to the London cities for managing routes.
+          @london_cities = @london_zoomed.flat_map { |hex| hex.tile.cities }
         end
 
         # Keeps the zoomed London hex colour and city revenue in sync with the